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Propyne (CH3-C≡C-H) is treated separately with each of the following reagents. Which reagent gives a reaction that is specific to terminal alkynes (i.e., gives a positive test only with a terminal alkyne)?

1. Br2 / H2O
2. Cold KMnO4 in basic medium (OH-)
3. Ammoniacal silver nitrate solution [Ag(NH3)2]+
4. Tetrammine copper(II) chloride [Cu(NH3)4]Cl

Correct answer: Ammoniacal silver nitrate solution [Ag(NH3)2]+

Solution

The acidic terminal C-H of a terminal alkyne reacts with Ag+ ions in ammoniacal solution to form an insoluble silver acetylide precipitate (white), a test specific to terminal alkynes. Br2/H2O and cold KMnO4 also react with internal alkynes and are not specific.

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